Vibrationally controlled chemistry:: Mode- and bond-selected reaction of CH3D with CI

Selective vibrational excitation controls the competition between C-H and C-D bond cleavage in the reaction of CH3D) with Cl, which forms either HCl + CH2D) or DCl + CH3. The reaction of CH3D molecules with the first overtone of the C-D stretch (2 nu(2)) excited selectively breaks the C-D bond, producing CH3 exclusively. In contrast, excitation of either the symmetric C-H stretch (vi), the antisymmetric C-H stretch (nu(4) or a combination of antisymmetric stretch and CH3 umbrella bend (nu(4) + nu(3)) causes the reaction to cleave only a C-H bond to produce solely CH2D. Initial preparation of C-H stretching vibrations with different couplings to the reaction coordinate changes the rate of the H-atom abstraction reaction. Excitation of the symmetric C-H stretch (nu(1)) of CH3D) accelerates the H-atom abstraction reaction 7 times more than excitation of the antisymmetric C-H stretch (nu(4)) even though the two lie within 80 cm(-1) of the same energy. Ab initio calculations and a simple theoretical model help identify the dynamics behind the observed mode selectivity.